1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus and method for an internal combustion engine, and an engine control unit for purifying nitrogen oxides (NOx) within exhaust gases which flow through an exhaust passage by a NOx selective reduction catalyst under the existence of a reducing agent.
2. Description of the Prior Art
The type of conventional exhaust gas purifying apparatus for an internal combustion engine mentioned above is known, for example, from Japanese Patent Publication No. 6-35816. This exhaust gas purifying apparatus injects ammonia as a reducing agent to a NOx selective reduction catalyst from an upstream location of an exhaust passage. This exhaust gas purifying apparatus, which is applied to a diesel engine equipped in a vehicle, comprises an ammonia production unit for producing ammonia; an ammonia supply unit for supplying produced ammonia to a location in the exhaust passage upstream of the NOx selective reduction catalyst; an ECU for controlling these components; and a NOx sensor disposed at a location of the exhaust passage upstream of the Nox selective reduction catalyst and connected to the ECU. The ammonia production unit, which generates ammonia from light oil and air, comprises a catalytic cracking reactor, an oxidization reactor, an ammonia synthesis reactor, and the like.
The ammonia supply unit in turn comprises an ammonia cylinder for storing ammonia produced by the ammonia production unit; and a control valve for controlling the amount of ammonia supplied from the ammonia cylinder to the exhaust passage. In this exhaust gas purifying apparatus, the ECU determines the amount of supplied ammonia necessary for NOx purification based on the NOx concentration in exhaust gases detected by the NOx sensor, and controls the opening of the control valve based on the determined amount of supplied ammonia.
The foregoing exhaust gas purifying apparatus employs the NOx selective reduction catalyst because it suffers less from sulfur poisoning and is less susceptible to the degradation of reduction performance, as compared with a NOx adsorption catalyst or a NOx occlusion catalyst, as well as because it eliminates a rich spike control for enriching the air/fuel ratio of an air-fuel mixture to recover the NOx adsorption performance (purification performance) of the catalyst to permit an increase in fuel economy.
In the conventional exhaust gas purifying apparatus described in Japanese Patent Publication No. 6-35816, since the NOx sensor is disposed in the exhaust passage at a location upstream of the NOx selective reduction catalyst, the amount of ammonia supplied to the NOx selective reduction catalyst is simply determined based on the NOx concentration of exhaust gases discharged from a diesel engine irrespective of how NOx is actually purified by the NOx selective reduction catalyst. As a result, the amount of ammonia supplied to the NOx selective reduction catalyst is not appropriately determined in accordance with the actual purification of NOx by the NOx selective reduction catalyst, possibly exacerbating the exhaust gas characteristics of the engine.
To solve the problem mentioned above, it is contemplated that the NOx sensor is positioned in the exhaust passage at a location downstream of the NOx selective reduction catalyst, such that the amount of ammonia supplied to the NOx selective reduction catalyst is determined on the basis of the NOx concentration detected by thus positioned NOx sensor. However, it has been generally confirmed in experiments that the Nox sensor characteristically reacts not only with NOx in exhaust gases but also with ammonia. Therefore, as an increase in ammonia not consumed in the NOx reduction causes a higher concentration of ammonia in exhaust gases downstream of the NOx selective reduction catalyst, the NOx sensor, if positioned in the exhaust passage at a location downstream of the NOx selective reduction catalyst, detects a value which apparently indicates a high NOx concentration. As a result, since the NOx sensor fails to correctly detect the NOx concentration in exhaust gases, the ECU does not determine an appropriate amount of ammonia supplied to the NOx selective reduction catalyst, possibly resulting in exacerbated exhaust gas characteristics of the engine.